Internal combustion engines may include central fuel injection (CFI) systems that inject fuel into an intake manifold. When fuel is injected into the engine intake, heat is transferred from the intake air and/or engine components to the fuel and this heat transfer leads to atomization of a portion of the fuel, which results in cooling of the engine components. Injecting fuel into the intake air (e.g., in the intake manifold, ports, etc.) lowers both the intake air temperature and a temperature of combustion at the engine cylinders. By cooling the intake air charge, NOx production may be reduced. NOx is stored in an exhaust catalyst which is periodically or opportunistically regenerated during richer than stoichiometric air fuel ratio operation. In addition to CFI, fuel may be injected to intake runners via port injectors and/or directly into cylinders via direct injectors.
Various approaches are provided for reducing production of NOx during engine operation. One example approach is shown by Mulye in U.S. Pat. No. 8,935,996, wherein liquid water is injected into engine cylinders during compression and power strokes to reduce the temperature of combustion at the cylinders. As the combustion temperature is reduced, there is a reduction in NOx emissions. The water is supplied to the cylinders via dedicated water injectors coupled to an on-board water source.
However, the inventors herein have recognized potential issues with such systems. As one example, increase in the amount of vaporized water in the combustion chambers, due to the water injection, may result in combustion instability. Also, there may be conditions where water may not be readily available on-board a vehicle for injection, such as due to ambient conditions or engine operating conditions not being conducive to water generation on-board the vehicle, or due to water consumption during a previous drive cycle having exceeded water generation. The water shortage may lead to an increased NOx level in the exhaust during engine operation. As another example, during engine cold-start conditions, before an exhaust catalyst attains the light-off temperature, exhaust NOx may not be effectively captured and consequently there may be an increase in NOx emissions level. Only after the exhaust catalyst has attained the light-off temperature is the exhaust catalyst able to adsorb oxidants such as NOx and oxygen passing there-through. As a further example, during conditions such as deceleration fuel shut-off (DFSO) when cylinder fueling is deactivated while air flows through the engine, the exhaust catalyst may become saturated with oxygen such that upon resumption of cylinder fueling, further adsorption of NOx may be limited until the engine is operated with a richer than stoichiometric air-fuel ratio to purge the catalyst. As a result, emissions quality may be adversely affected. In addition, the need for additional fueling to purge the catalyst adversely affects fuel economy.
In one example, the issues described above may be addressed by a method comprising: adjusting a first portion of fuel delivered to an engine via manifold injection relative to a second portion of fuel delivered to the engine via one or more of port and direct injection based on an estimated oxygen content of an exhaust catalyst, the estimated oxygen content estimated immediately after a fuel shut-off event. In this way, by injecting a portion of the total mass of fuel to be injected via central fuel injection, combustion temperatures may be reduced, thereby reducing NOx production.
As one example, during engine cold-start conditions, an engine controller may determine an initial fuel injection profile based on engine operating conditions such as engine speed and engine load. The initial fuel injection profile may include an amount of fuel to be delivered via manifold fuel injection (e.g., via a central manifold fuel injector or CFI), and a remaining amount of fuel to be delivered via one or more of port and direct fuel injection. As the fuel injected via the CFI atomizes in the intake manifold, the intake manifold may be cooled, creating a local charge cooling effect. During higher than threshold engine intake temperatures, the portion of fuel supplied via CFI may be increased until the intake temperature decreases to below the threshold temperature. Upon exit from a DFSO event, a richer than stoichiometric air fuel ratio may be maintained to desorb the NOx trapped in the exhaust catalyst and then to convert the NOx to water and nitrogen under the richer conditions. A portion of fuel delivered during the richer than stoichiometric engine operation may be provided via CFI. Also, if the catalyst is saturated with oxygen during the DFSO event, the amount of fuel supplied via CFI may be increased until the oxygen content in the catalyst decreases to below the threshold content.
In this way, during conditions conducive to production of a higher amount of NOx, by opportunistically injecting fuel via CFI, charge cooling may be increased and NOx production may be correspondingly decreased. By using manifold injected fuel for charge cooling, dependence on variably available water sourced on-board the vehicle may be reduced. By increasing manifold charge cooling during an engine cold-start, NOx production may be reduced during conditions when an exhaust catalyst is not active and is not able to optimally trap NOx. The technical effect of injecting fuel via CFI while operating at a richer than stoichiometric air-fuel ratio upon exit from a DFSO condition is that further production of NOx may be lowered. By increasing charge cooling and reducing NOx production during conditions when the catalyst is saturated with oxygen, emissions quality may be improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.